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Metal-Catalyzed Anaerobic Disproportionation of Hydroxylamine. Role of Diazene and Nitroxyl Intermediates in the Formation of N2, N2O, NO+, and NH3

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journal contribution
posted on 20.10.2004, 00:00 by Graciela E. Alluisetti, Alejandra E. Almaraz, Valentín T. Amorebieta, Fabio Doctorovich, José A. Olabe
The catalytic disproportionation of NH2OH has been studied in anaerobic aqueous solution, pH 6−9.3, at 25.0 °C, with Na3[Fe(CN)5NH3]·3H2O as a precursor of the catalyst, [FeII(CN)5H2O]3-. The oxidation products are N2, N2O, and NO+ (bound in the nitroprusside ion, NP), and NH3 is the reduction product. The yields of N2/N2O increase with pH and with the concentration of NH2OH. Fast regime conditions involve a chain process initiated by the NH2 radical, generated upon coordination of NH2OH to [FeII(CN)5H2O]3-. NH3 and nitroxyl, HNO, are formed in this fast process, and HNO leads to the production of N2, N2O, and NP. An intermediate absorbing at 440 nm is always observed, whose formation and decay depend on the medium conditions. It was identified by UV−vis, RR, and 15NMR spectroscopies as the diazene-bound [FeII(CN)5N2H2]3- ion and is formed in a competitive process with the radical path, still under the fast regime. At high pH's or NH2OH concentrations, an inhibited regime is reached, with slow production of only N2 and NH3. The stable red diazene-bridged [(NC)5FeHNNHFe(CN)5]6- ion is formed at an advanced degree of NH2OH consumption.